The third annual March Against Monsanto was being held in upwards of 400 cities in more than 40 countries.
About 2,500 people staged anti-Monsanto protests in the Swiss cities of Basel and Morgues, where the company has its headquarters for Europe, Africa and the Middle East.
Up to 3,000 protesters rallied by environmental organisations including Greenpeace and anti-capitalist group Stop Tafta gathered in Paris, with Monsanto's market-leading herbicide Roundup the main targets of protesters' anger.
Halfway around the planet in Burkina Faso, around 500 marched against the US giant which introduced GM cotton into the west African country in 2003.
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Demonstrators demanded a 10-year moratorium on the planting of Monsanto seeds so "independent research can be conducted" into the effects of the technology.
Up to 1,000 anti-Monsanto activists gathered in front of the European Parliament in Strasbourg as the sun was setting for a minute's silence "in homage to the existing and future victims poisoned by pesticides", according to the organisers.